The present invention relates to an aqueous slurry of calcium carbonate particles and a method for the preparation thereof. More particularly, the invention relates to an aqueous slurry containing precipitated calcium carbonate particles and ground calcium carbonate particles in combination and exhibiting an outstandingly low high-shear viscosity and excellent high-speed flowability despite the relatively high solid concentration so as to be useful as a base composition for the preparation of a paper coating composition capable of giving coated paper sheets, including coated paper sheets, such as art paper, general-purpose coated paper, light-coated paper, medium-coated paper and reclaimed coated paper, thermosensitive recording paper, pressure-sensitive recording paper, ink-jet recording paper, PPC paper and thermotransfer paper, exhibiting excellent printability and printing glossiness as printed as well as a method for the preparation of such an aqueous slurry of calcium carbonate particles.
In recent years, production of coated paper products is under growing year by year and the growth of production is particularly remarkable for coated paper products of A2, A3, B2 and B3 grades as well as light weight-coated paper.
Along with the progress in paper-making machines and paper-coating machines for a higher and higher running velocity in recent years, technology innovation is now under way for blade coaters as a type of paper-coating machines. Accordingly, while the running velocity of conventional paper coating machines is around 1000 to 1500 meters/minutes, a higher running velocity exceeding 1500 meters/minute is now within view for modern paper-coating machines to be newly installed.
Needless to say, such a high running velocity of the paper coating process cannot be accomplished by the progress of the paper-coating machines only but improvements of the paper coating compositions are indispensable relative to the rheological properties such as lower viscosity or consistency and higher flowability to comply with the high-speed running of the paper-coating machine as well as to the solid concentration thereof which should be as high as possible in order to decrease the load on the drying machines while the requirements for the lower viscosity and higher solid concentration in an aqueous paper coating composition are sometimes contradictory.
In the paper coating process by using a blade coater mentioned above, the paper coating composition applied to the surface of the running base paper web is subjected to a large shearing force so that poor Theological properties of the paper coating composition results in appearance of striations in the coating layer on the coated paper sheet. Accordingly, a primary requisite for a paper coating composition used in a blade coater is that the coating composition has a low viscosity even under a high shearing force, i.e. high-shear viscosity, to exhibit good high-speed flowability even when the solid concentration thereof is relatively high.
As is known, an aqueous paper coating composition for coated paper is an aqueous slurry of an inorganic pigment, such as clay, e.g., kaolin clay, precipitated calcium carbonate, ground calcium carbonate, titanium dioxide and satin white either alone or as a combination of two kinds or more, dispersed in an aqueous medium containing a water-soluble resin as a binder of the pigment particles together with other optional additives. Since a paper coating composition containing kaolin clay alone as the pigment is sometimes inferior in respect of the high-speed flowability, for example, it is a usual practice that a slurry of kaolin is admixed with a slurry of calcium carbonate particles to prepare a paper coating composition in order to improve the high-speed flowability of the composition.
A high solid concentration of a paper coating composition is usually accomplished by increasing the content of the pigment in the pigment slurry. Pigment slurries typically employed in the preparation of a paper coating composition include a slurry of kaolin clay having a solid concentration of 60 to 75% by weight prepared by cowless dispersion, a slurry of dry-ground calcium carbonate further ground in a wet process having a solid concentration of 70 to 80% by weight and a slurry having a solid concentration of 60 to 70% by weight prepared by dispersing a dry powder or wet cake of precipitated calcium carbonate in water.
The formulation of paper coating compositions relative to the pigment is, though dependent on the grade of the coated paper product, type of the base paper web, desired coating amount and other factors, usually in the range from 50 to 95% by weight for a clay pigment and from 5 to 50% by weight for a calcium carbonate pigment. The calcium carbonate pigment is selected depending on the intended application field and grade of the coated paper product from ground calcium carbonate, precipitated calcium carbonate and mixtures of these two types of calcium carbonate powders in an appropriate mixing proportion.
Clay pigments are graded into several classes including class 1, class 2, delamination class and the like depending on various factors such as place of occurrence as a natural mineral and the like. It is usual that a paper coating composition is prepared by using the class 1 and class 2 clay pigments in combination.
Ground calcium carbonate pigments are graded into #95, #90, #75 and #60 for wet-process slurries and into #2200, #2000, #1500, #800 and #100 for dry-ground powders. It is usual that a paper coating composition for under-coating is prepared using a coarse-grade calcium carbonate pigment and top-coating composition is prepared using a fine-grade pigment. In particular, paper coating compositions used in high-speed coating machines are usually prepared from a slurry of 70 to 80% solid concentration of wet-ground calcium carbonate of the #95 and #90 grades for top coating and wet-ground calcium carbonate of #75 and #60 grades for under-coating.
Precipitated calcium carbonate pigments are sold on the market in the form of an aqueous slurry and dry powder having different particle sizes in which the particles have a scalenohedral, i.e. rosette-shaped or spindle-formed, rhombohedral, prismatic or acicular configuration. Aqueous slurries of 60 to 70% solid concentration having somewhat improved high-speed flowability characteristics are usually employed in a paper coating composition for the preparation of coated paper of the A2, A3, B2 and B3 grades and light weight-coated paper as coated printing paper.
When the whiteness of the base paper is relatively low, when intended coating amount is small, when a high glossiness is desired or when good printability of the coated paper is essential, it is a preferable way to use a precipitated calcium carbonate pigment alone, in many cases, in combination with a ground calcium carbonate pigment because precipitated calcium carbonate pigments well meet these requirements.
An aqueous slurry of a precipitated calcium carbonate pigment, however, has a problem, as compared with ground calcium carbonate pigments, that the slurry of the former pigment is inferior in the high-speed flowability behavior when the solid concentration is identical so that a paper coating composition prepared from a precipitated calcium carbonate pigment can hardly be used in high-speed coating machines.
In view of the above described problem, a proposal has been made in Japanese Patent Kokai 57-184430, in order to obtain an aqueous slurry of high solid concentration from a precipitated calcium carbonate pigment, for a method in which an aqueous slurry of 60 to 80% by weight solid concentration is prepared by using a combination of two powders of precipitated calcium carbonate having different particle diameters with admixture of an inorganic electrolyte such as calcium hydroxide. In this method, for example, a viscosity-reducing agent and calcium hydroxide are added and dispersed in an aqueous slurry of a precipitated calcium carbonate pigment followed by additional admixture of a powder of precipitated calcium carbonate and calcium hydroxide to make up a slurry of 70 to 72% solid concentration. The aqueous slurry prepared by this method is naturally expensive because of the use of a dry calcium carbonate powder as an adjuvant and the high-shear viscosity is as high as 230 to 490 centipoise due to the mixing process by using a stirrer.
A method is proposed in Japanese Patent Kokai 54-120709, according to which a calcium carbonate slurry suitable for paper quality improvement is prepared by introducing a powder of ground calcium carbonate and slurry of precipitated calcium carbonate into a high-speed mixer and intensively mixing the blend. The slurry obtained by this method, however, has a disadvantage in respect of the low paper quality such as white paper glossiness and a slurry of a high solid concentration of 75% by weight or higher can hardly be obtained as compared with slurries of wet-ground calcium carbonate currently under practical use in many paper manufacturers.
With an object to improve the high-speed flowability of an aqueous slurry of ground calcium carbonate and to upgrade the quality of the coated paper such as white-paper glossiness to be comparable with that by using a slurry of precipitated calcium carbonate, a proposal has been made in Japanese Patent Publications 55-11799, 1-15640, 4-33731 and 6-45010 and Japanese Patent Kokai 57-30555 and 63-225522 according to which a dry-ground calcium carbonate powder is subjected to wet-grinding in a sand grinder. This method has been developed so far as to furnish an aqueous slurry of wet-ground calcium carbonate having a solid concentration of 70 to 75% by increasing the degree of grinding to accomplish a finer average particle diameter with a decreased amount of coarser particles resulting in an improvement in the white-paper glossiness of coated paper products.
When comparison is made for a slurry of wet-ground calcium carbonate of #90 or #95 grade with that of precipitated calcium carbonate relative to the quality of coated paper, however, the coated paper is generally inferior in the whiteness, white-paper glossiness and printability such as ink receptivity and ink setting though equivalent printing glossiness can be obtained. As a consequence of the fine grinding, in addition, the BET specific surface area of the powder is increased so much necessitating an increase in the amount of the dispersing agent with an increase in the high-shear viscosity and decrease in the high-speed flowability as well as a decrease in the surface strength of the coated paper which requires an increase in the amount of the binder in the preparation of paper coating compositions.